In order to determine the distance between two stations, the time taken by radio waves to travel from an interrogating to a responding station and back is measured at the interrogating station.
In certain installations, the radio signal emitted by the interrogating station is simply reflected back by the responding station.
In other installations, the radio wave emitted by the interrogating station triggers, on its arrival at the other station termed a transponder, the emission of radio signals traveling in the opposite direction. In these latter installations, the accuracy of the measurement depends on the constancy of the time elapsing between the arrival of the radio signals emitted by the interrogating station at the transponder station and the retransmission of the response signals by the transponder.
However, the electronic transponder components provided for receiving and treating the incoming signals and for controlling the retransmission have characteristics which vary with time, with temperature or with other factors. Thus, even the use of components of high quality does not make it possible to eliminate the resulting variation in the intervening time lag.
This problem is aggravated by the fact that the signals received at the transponder station are affected by noise so that, if retransmission is to be carried out with relatively low energy, it is necessary to use transponder components with a narrow bandwidth, which are particularly sensitive to interference factors.